I want to design a passive 4-pole low-pass RLC filter for my subwoofer amplifier. So far I found this page: http://sim.okawa-denshi.jp/en/RLClowkeisan.htm But it seems that changing the value of R does nothing to the cutoff frequency. I think this is strange and would like to clarify this.

Also, I'm confused about the impedances to consider when designing low-pass filters. Do I include the source impedance only? PC Sound Card output impedance is 50 Ohms I guess? Or do I also include the load impedance, which in my case is the amplifier input, and is 30k? I'm confused.

I'd like a good (true) online calculator or a design tool for 4-pole RLC filters, or at least an explanation.

I forgot to mention this, but I'm planning to cascade the filters through an amplifier. This is different than using an active filter on opamps, but I work with what I have. Basically I have a stereo amp and one of its channels I'm going to use for amplifying between the two 2-pole filters. Still I dont get how the linked page suggests the cutoff frequency is same for different impedances. Somehow I don't think I can use the same micro filter after the final amp output where the signal is powerful and load impedance is low. Intuitively, I'd need much bigger inductors and capacitors. The page doesn't reflect that

  • \$\begingroup\$ You forgot to provide important information, like the values that you put into the calculator. The R changes the filter for me, and it should. You are either misreading the graph, or you have two parameters that are significantly larger. \$\endgroup\$
    – Voltage Spike
    Mar 2 '16 at 5:11
  • \$\begingroup\$ @laptop2d Try RC filter with R=680 and C=4.7u. Cut-off frequency is 50Hz. I tested this circuit with a PC frequency response analyzer, and it indeed was cutting from 50Hz. Then try a RLC filter with R=680, C=4.7u and L=0.000001p. It says the cut-off frequency is 73412700957.2[Hz]. Somehow I don't think this is true. I also tested the circuit with 1uH (what I had on hand), and guess what - even a larger inductance than 0.000001p had no visible effect on the filter (it was behaving basically like an RC filter, which is expected) \$\endgroup\$
    – Rikudou
    Mar 4 '16 at 15:39
  1. The circuit shown at the web page you linked to is only a 2-pole filter. If you try to make a 4-pole filter by cascading two such 2-pole filters, the performance is likely to be not as expected due to loading effects. Basically this means that the input impedance of this circuit is not constant across frequency, so when you cascade filters together you need to adjust the first filter to accomodate the changing load presented by the second filter.

  2. It is correct that changing R does not change the filter characteristic frequency, it only affects the damping ratio. This is expected behavior.

If you want to design a 4-pole passive filter, I suggest using LTSpice and tuning the filter until you achieve good performance accounting for loading effects. If you want to just use simple calculations, then you could switch to active filters or provide a buffer between your 2-pole filter sections to eliminate loading (assuming your power handling requirements allow this).

If you really need to design this as a 4-pole passive filter, then you can go back to pre-1970's filter design textbooks or cookbooks to find the design techniques that were used before numerical analysis and active filter designs became ubiquitous.

  • \$\begingroup\$ 1. You are right, I am going to use an intermediate amplifier between the filters. 2. This doesn't make sense. Shouldn't an RLC circuit with very small L-values behave almost like an RC-circuit? The RC circuit calculator on the said site behaves very differently than the RLC calculator with very small L's. \$\endgroup\$
    – Rikudou
    Mar 2 '16 at 15:41
  • \$\begingroup\$ @Rikudou, the simulator and the way the results are displayed probably assume when you make an RLC filter the R will be a small parasitic and you are trying to make a 2-pole filter; and when you ask for an RC filter you are trying to make a 1-pole filter. No matter how small the L in the RLC filter, it will contribute a 2nd pole, but it may be at a very high frequency. \$\endgroup\$
    – The Photon
    Mar 5 '16 at 16:33
  • \$\begingroup\$ after some more research on the topic I've decided to drop the idea of using inductors. It seems that the inductance needed is only reasonably small for low load impedance values, such as speaker drivers. For a load like an amplifier input I'd need some absurdly high inductances, so active opamp RC filters looks like the only reasonable idea. Sigh.. \$\endgroup\$
    – Rikudou
    Mar 6 '16 at 12:11

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